Exhaust gas treatment method and device

ABSTRACT

A method of treating NOx emmissions in the exhaust gas of an internal combustion engine having catalyst means including at least a first catalyst converter capable of treating NOx, the method including operating the engine in a first mode to promote a first set of conditions and in a second mode to promote a second set of conditions, wherein the first mode of operation includes operating the engine with a lean air-fuel ratio, and the second mode of operation includes operating the engine with a stoichiometic air-fuel ratio.

This invention relates to the treatment of oxides of nitrogen within theexhaust gas emissions of internal combustion engines, and in particularto a method of operating an internal combustion engine to allow suchtreatment.

The recent and future introduction of increasingly strict internalcombustion engine emissions legislation around the world, particularlyas this relates to automotive vehicles, has resulted in increasingpressure on engine and vehicle manufacturers to reduce engine emissions,particularly hydrocarbon (HC), carbon monoxide (CO), and oxides ofnitrogen (NOx) emissions. These emissions are generally treated by acatalytic converter in the exhaust system of the engine, which isintended to convert these potentially harmful gases into preferredsubstances such as carbon dioxide, nitrogen, oxygen, and water.

NOx emissions present particular challenges for engine and vehiclemanufacturers in that typical catalytic converters have been found to beless effective when the engine is operating under lean burn conditions.This is particularly a problem in engines which derive efficiencyadvantages from lean burn operation, and in particular, stratifiedcharge engines, such as some of those incorporating the Applicant's dualfluid fuel injection system.

Dual fluid fuel injection systems typically utilise compressed gasduring each injection event to entrain and atomise a metered quantity offuel for delivery into the combustion chambers of an internal combustionengine. The Applicant has developed such fuel injection systems and oneversion thereof is described in the Applicant's U.S. Pat. No. 4,934,329,the details of which are incorporated herein by reference. Generally, asource of compressed gas, for example an air compressor, is required forthese fuel injection systems to operate satisfactorily. The term “air”is used herein to refer not only to atmospheric air, but also to othergases including air and exhaust gas or fuel vapour mixtures. Inoperation, such dual fluid fuel injection systems typically rely on theexistence of a differential pressure between the fuel which is meteredfor subsequent delivery and the compressed gas, typically air, which isused to deliver the fuel to the engine. In this regard, it is normalthat the fuel pressure is slightly higher than the air pressure suchthat the fuel may be metered into a volume of compressed gas in a mannerakin to that described in U.S. Pat. No. 4,934,329.

PRIOR ART

Various methods of engine operation and engine exhaust systems have beenproposed to overcome the problem of NOx emissions. One known example,set out in U.S. Pat. No. 5,433,074, proposes the use of a specific NOxadsorbent layer in the catalyst. This layer or coating is intended toabsorb NOx emissions under typical low NOx conversion conditions (thatis, during lean burn operation of the engine) and release the absorbedNOx under typical high NOx conversion conditions (that is, during richerthan stoichiometric operation of the engine). The adsorbent layer is aNOx adsorbent material including Barium (Ba).

However fuel economy in a system utilising such catalysts can be itcompromised by the requirement of periodic “flushing” of the system witha rich air-fuel mixture. Further, in order to ensure effective operationof the system, additional sensors may be required to provide feedback tothe engine controller for the purpose of determining whether “flushing”is required. The system may also be temperature sensitive, and damage tothe adsorbent layer may occur at temperatures above 750-degrees Celsius,whilst effective operation of the storage capacity may be limited to awindow of around 300 to 550 degrees Celsius.

SUMMARY OF THE INVENTION

It is the aim of this invention to provide an alternative NOx treatmentmethod and device, which overcomes at least some of the disadvantages ofthe prior art systems.

In accordance with a first aspect of the present invention, there isprovided a method of treating NOx emissions in the exhaust gas of aninternal combustion engine having catalyst means including at least afirst catalyst converter capable of treating NOx, the method includingoperating the engine in a first mode to promote a first set ofconditions and in a second mode to promote a second set of conditions,wherein the first mode of operation includes operating the engine with alean air-fuel ratio, and the second mode of operation includes operatingthe engine with a stoichiometric air-fuel ratio, the method furtherincluding controlling the operation of the engine during the first modeso as to promote a selective catalyst NOx reduction process at the firstcatalytic converter.

Conveniently, the catalyst means includes a first catalyst converterarranged in an exhaust system of the engine. Preferably, the first setof conditions include exhaust gases with lean air-fuel ratio and lowerrelative temperatures. Conveniently, the second set of conditionsinclude exhaust gases with a stoichiometric air fuel ratio. In manycases, the second set of conditions will include higher relative exhaustgas temperatures. Preferably, the exhaust gas temperatures produced bythe engine while it operates under the first mode of operation are inthe range 200 to 400 degrees Celsius. Preferably, the exhaust gastemperatures produced by the engine while it operates under the secondmode of operation are greater than 200 degrees Celsius, and typicallythe exhaust gas temperature are greater than 400 degrees Celsius.Preferably the relevant exhaust temperature is that of the exhaust gasat the first catalytic converter. Preferably the temperature of theexhaust gas is controlled by way of appropriate operation of the engineto ensure effective operation of the first catalytic converter under thefirst mode of operation. Preferably the temperature of the exhaust gasin this case is controlled to be within the range 200 to 400 degreesCelsius. Preferably the temperature of the exhaust gas is controlled byway of appropriate operation of the engine to ensure effective operationof the first catalytic converter under the second mode of operation.Preferably the temperature of the exhaust gas In this case is to begreater than approximately 400 degrees Celsius. Conveniently, theoperation of the engine is controlled during the first mode so as togenerate the exhaust gas emissions having characteristics that cansupport acceptable levels of _(Nox) conversion within the firstcatalytic converter.

Preferably the first catalytic converter includes a combination of Pt(or Pd), Rh and Ba elements. Preferably, the first catalytic convertercomprises a greater proportion of Pt (ie: it is “Pt rich”) than would beexpected in a typical three way catalyst. Preferably the ratio of Pt toRh in the first catalytic converter is 10:1. Preferably, the proportionof Ba in the first catalyst converter is relatively low as compared tothe proportions of Pt and Rh. The operation of the engine during thefirst mode is controlled so as to promote a selective catalyst reductionprocess at the first catalytic converter which is normally not supportedduring lean burn operation. The composition of the first catalyticconverter is preferably slightly different to that expected in a typicalthree way catalyst comprising pt (or Pd) and Rh. Conveniently, thesubtle difference in the composition of the first catalyst convertertogether with the promotion of the first set of conditions during thefirst mode enable the achievement of higher NOx emission efficienciesthan would otherwise be expected form a typical three way catalystduring the said first mode of operation.

Conveniently, the operation of the engine is controlled during thesecond mode so as to promote high NOx conversion efficiency levelswithin the first catalytic converter.

Preferably a temperature sensing device is provided in the exhaustsystem of the internal combustion engine, and the output from thetemperature sensing device is used to determine the mode of operation ofthe internal combustion engine. Preferably a sensed temperature ofbetween 200 and 400 degrees Celsius will result in operation of theengine under the first mode of operation. Preferably a sensedtemperature of greater than 400 degrees Celsius will result in operationof the engine under the second mode of operation. This latter mode ofoperation will typically equate to high engine load operating conditionswherein the temperatures of the exhaust gas are usually higher thanduring lean burn operation.

Preferably the first catalytic converter is provided in the exhaustsystem at a position sufficiently downstream of the internal combustionengine that the exhaust gas is allowed to cool somewhat before enteringthe first catalytic converter.

Preferably a second catalytic converter is provided in a close coupledconfiguration with the internal combustion engine for the purpose ofoxidising hydrocarbon and carbon monoxide emissions in the engineexhaust gases. Preferably the first catalytic converter is a three waycatalyst. Conveniently, the engine is direct injected. Preferably, fuelinjection to the engine is effected by way of a two fluid fuel injectionsystem,

According to another aspect of the present invention, there is providedan engine exhaust system for treating NOx emissions in the exhaust gasof an internal combustion engine, including catalyst means having atleast a first catalyst converter capable of treating NOx, wherein theengine exhaust system is adapted to at least selectively reduce aportion of the NOx emissions when the engine is operated in a first modeand a first set of conditions are promoted, and the first mode ofoperation includes operating the engine with a lean air-fuel ratio.

According to a further aspect of the present invention, there isprovided an electronic control unit for controlling an internalcombustion engine having catalyst means including at least a firstcatalyst converter capable of treating NOx, the electronic control unitoperating the engine in a first mode to promote a first set ofconditions and in a second mode to promote a second set of conditions,wherein the first mode of operation includes operating the engine with alean air-fuel ratio, and the second mode of operation includes operatingthe engine with a stoichiometric air-fuel ratio to thereby treat NOxemissions in the exhaust gas of the engine.

According to yet another aspect of the present invention, there isprovided an internal combustion engine for use with an exhaust treatmentsystem having reversible NOx adsorbent capability, said engine having afuel injection system which facilitates operation of said engine with aplurality of air fuel ratios in a range between lean and rich and saidengine having an electronic controller for controlling operation of saidengine and for selecting between said air fuel ratios, wherein saidselection is not directly dependent on the amount of NOx stored orcalculated to be stored in said exhaust treatment system.

According to a further aspect of the present invention, there isprovided an internal combustion engine and exhaust treatment system fora vehicle, said exhaust treatment system having reversible NOx adsorbentcapability, said engine having a fuel injection system which facilitatesoperation of said engine with a plurality of air fuel ratios in a rangebetween lean and rich and said engine having an electronic controllerfor controlling operation of said engine and for selecting between saidair fuel ratios, wherein the amount of NOx emitted by said engine tosaid exhaust treatment system over a Euro III drive cycle is no morethan four times the Euro III requirement whereby said exhaust treatmentsystem has emissions of NOx, carbon monoxide and hydrocarbons less thansaid Euro III requirement over said Euro III drive cycle.

According to another aspect of the present invention, there is providedan internal combustion engine for use with an exhaust treatment systemhaving reversible NOx adsorbent capability, said engine having a fuelinjection system which facilitates operation of said engine with aplurality of air fuel ratios in a range between lean and substantiallystoichiometric and said engine having an electronic controller forcontrolling operation of said engine and for selecting saidsubstantially stoichiometric air fuel ratio to purge NOx stored in saidexhaust go treatment system.

According to another aspect of the present invention, there is providedan internal combustion engine and exhaust treatment system for use in avehicle, said exhaust treatment system comprising at least one catalysthaving three way conversion capability and NOx storage capability,wherein the amount of NOx emitted by said engine to said exhausttreatment system over a Euro III drive art cycle is no more than fourtimes the Euro III requirement whereby said exhaust treatment system hasemissions of NOx, carbon monoxide and hydrocarbons less than said EuroIII requirement over said Euro III drive cycle, and the volume of thecatalyst is less than 150% of the swept volume of said engine.

PREFERRED EMBODIMENT OF THE INVENTION

It will be convenient to further describe the invention with respect tothe accompanying drawings that assist in describing various preferredembodiments of the present invention. Other embodiments of the inventionare however possible, and consequently, the particularity of theaccompanying drawings is not to be understood as superseding thegenerality of the preceding description of the invention.

In the drawings:

FIG. 1 is a schematic partial cross-sectional view of an internalcombustion engine having a dual fluid fuel injection system operativelyarranged with respect thereto;

FIG. 2 is a partial cross-sectional view of one form of a fuel meteringand injector rail unit;

FIG. 3 is a schematic layout of an internal combustion engine andexhaust system according to an embodiment of the present invention; and

FIG. 4 is a graph showing engine load against engine speed for an engineoperating in accordance with an embodiment of the present invention.

FIG. 5 is a flow chart describing how selection between the variousmodes of operation detailed in FIG. 4 may be effected.

As referred to above, emissions legislation is being introduced aroundthe world that requires engine and vehicle manufactures to reduce theemissions produced by various types of vehicles. An example of suchlegislation that is applicable to Europe is commonly referred to as theEuro III and Euro IV emissions targets and should be well known to thoseskilled in the relevant art

The Euro III and Euro IV emissions targets for passenger vehiclespowered only by gasoline in respect of HC, CO and NOx emissions are:

EC 2000 EC 2005 TEST EMISSIONS UNIT (EURO III) (EURO IV) Rev. HC g/km0.2 0.1 ECE + NOx 0.15 0.08 EUDC CO 2.3 1.0 Passenger Vehicles (≦2.5 tgross vehicle weight)

To make these measurements of vehicle emissions, a vehicle is typicallyoperated on a dynamometer. The dynamometer is caused to operate with aspecific drive cycle that simulates certain real world drivingconditions. Euro III and Euro IV have specific drive cycles over whichthe emissions referred to above are measured, these drive cycles arereferred to as the ECE and the EUDC drive cycles.

The emissions that are measured are referred to as tail pipe emissionsas they are emitted from the exhaust pipe (often referred to as the“tail pipe”) of the vehicle. In a typical vehicle, emissions from theengine (often referred to as “engine out” emissions) are treated by anexhaust treatment system that typically utilises a catalytic converterwhich promotes further reduction and oxidation of engine out emissionsso that the tail pipe emissions contain a greater proportion of N₂, O₂,CO₂, and H₂O than the engine out emissions. Hence the Euro III and EuroIV emissions specify maximum levels of tail out emissions ofhydrocarbons, carbon-monoxide and oxides of Nitrogen for various classesof vehicles.

It is preferable that in meeting these emissions targets that thevehicle also have a fuel economy benefit over currently available MPI(Manifold Port Injected) engines and DI (Direct Injection) engines.

The Applicant has developed certain engine which utilize a two fluiddirect fuel injection system. Simple application of such fuel injectionsystems to four stroke engines is not, in itself, sufficient to meetthese emissions targets and further refinement is required before theabove emissions targets can be met. In particular it is necessary tocalibrate an engine at various points on the speed load curve (forexample the speed load curve detailed in FIG. 4) in order for it to meetthese emission targets. Calibration however is a multi-variable,typically non-linear problem. In a direct injection engine particularly,it involves consideration of variables such as ignition timing, fuel percycle, air fuel ratio, exhaust gas re-circulation levels, injectiontimings etc.

To fully understand how these emissions targets may be met by use ofsuch a fuel system, the Applicant's two fluid fuel injection system willfirst be described in some detail with reference to FIGS. 1 and 2, andthen a description of the application of the present invention to anengine with that fuel injection system will follow with particularreference to FIGS. 3 and 4. However, it is believed that application ofthe present invention need not be limited to engines with the describedfuel supply system, which it will be understood is set out for thepurposes of exemplification only. It may also be applicable to otherengines with similar emissions capabilities as the applicants engines.

FIG. 1 shows a direct injected four stroke internal combustion engine 20comprising a fuel injection system, the engine 20 having an air intakesystem 22, an ignition means 24, a fuel pump 23, and fuel reservoir 28.An air compressor 29 is operatively arranged with respect to the engine20 and typically driven off the engine crankshaft 33 or otherdrive-train by way of a suitable belt (not shown). Mounted in thecylinder head 40 of the engine 20 is a fuel and air rail unit 11. Thefuel pump 23 draws fuel from the fuel reservoir 28 which is thensupplied to the fuel and air rail unit 11 though a fuel supply line 55.Conventional inlet and exhaust valves 15 and 16 are also mounted in thecylinder head 40 in the known manner together with conventional cammeans 17 for actuating the valves 15, 16. The valves 15, 16 are arrangedto open and close corresponding inlet and exhaust ports 18 and 19 foradmission of fresh air and the removal of exhaust gases from the enginecylinder in the known manner.

Referring now to FIG. 2, there is shown in detail a fuel and air railunit 11 which, whilst being different in design from that shown in FIG.1, shares all the same components thereof. The fuel and air rail unit 11comprises a fuel metering unit 10 and an air or delivery injector 12 forthe or each cylinder of the engine 20. The fuel metering unit 10 iscommercially available and requires no detailed description herein.Suitable ports are provided to allow-fuel to flow through the fuelmetering unit 10 and a metering nozzle 21 is provided to deliver fuel toa passage 90 and thence to the air injector 12. The body 8 of the fueland air rail unit 11 may be an extruded component with a longitudinallyextending air duct 13 and a fuel supply duct 14.

As best seen in FIG. 1, at appropriate locations, there are providedconnectors and suitable ducts communicating the rail unit 11 with airand fuel supplies: air line 49 communicating air duct 13 with the aircompressor 29; air line 53 providing an air outlet which returns air tothe air intake system 22; and fuel line 52 communicating the fuel supplyduct 14 the fuel reservoir 28 providing a fuel return passage. The airduct 13 communicates with a suitable air regulator 27 which regulatesthe air pressure of the compressed air provided by the air compressor 29to the air duct 13.

Referring again to FIG. 2, the air injector 12 has a housing 30 with acylindrical spigot 31 projecting from a lower end thereof, the spigot 31defining an injection port 32 communicating with passage 90. Theinjection port 32 includes a solenoid operated selectively openablepoppet valve 34 operating in a manner similar to that as described inthe Applicant's U.S. Pat. No. 4,934,329, the contents of which arehereby incorporated by reference. As best seen in FIG. 1, energisationof the solenoid in accordance with commands from an electronic controlunit (ECU) 100 causes the valve 34 to open to deliver a fuel-gas mixtureto a combustion chamber 60 of the engine 20. However, it is not intendedto limit the valve construction to that as described above and othervalves, for example, pintle valve constructions, could be employed. Theelectronic control unit (ECU) 100 typically receives signals indicativeof crankshaft speed and airflow from suitably located sensors within theengine (not shown). The ECU 100, which may also receive signalsindicative of other engine operating conditions such as the enginetemperature, ambient temperature and battery voltage (not shown),determines from all input signals received the quantity of fuel requiredto be delivered to each of the cylinders of the engine 20. As alluded tohereinbefore, this general type of ECU is well known in the artelectronically controlled fuel injection systems and will not bedescribed herein further detail.

The opening of each injector valve 34 is cot lied by the ECU 100 via arespective communicating means 101 in timed relation to the engine cycleto effect delivery of fuel from the injection port 32 to a combustionchamber 60 of the engine 20. By virtue of the two fluid nature of thesystem, fuel is delivered to the cylinder entrained in a gas. Thepassage 90 is in constant communication with the air duct 13 via theconduit 80 as shown in FIG. 2 and thus, under normal operation, ismaintained at a substantially steady air pressure. Upon energization ofthe solenoid of air injector 12, the valve 34 is displaced downwardly toopen the injection port 32 so that a metered quantity of fuel deliveredinto the air injector 12 by the fuel metering unit 10 is carried by airthrough the injection port 32 in the combustion chamber 60 of a cylinderof the engine 20.

Typically, the air injector 12 is located within the cylinder head 40 ofthe engine 20, and is directly in communication with the combustionchamber 60 defined by the reciprocation of a piston 61 within the enginecylinder. As above described, when the injection port 32 is opened andthe air supply available via the conduit 80 is above the pressure in theengine cylinder, air will flow from the air duct 13 through the passage80, passage 90 and, entrained with fuel, injection port 32, into theengine combustion chamber 60.

Turning now to FIG. 3, a new set of reference numerals have been adopteddue to the schematic nature of this illustration. The featuresillustrated include engine 200, fuel intake 202, air intake 204, closecoupled catalytic converter 206, main catalytic converter 208 andexternal exhaust outlet 210. A temperature sensor 214 is locatedadjacent the entry to the main catalytic converter 208.

As is usual in the operation of engine systems of this type, fuel andair are taken in through their respective intakes 202, 204. Combustionthen takes place in the engine 200, and exhaust gases pass out of theengine 200. In this Figure, there is illustrated an optimal coupledcatalytic converter 206 through which the exhaust gases may passimmediately as they leave the combustion chamber of the engine 200.Exhaust gases then travel along exhaust pipe 212 to the main a catalyticconverter 208, and subsequently out the external exhaust outlet 210. Thecatalytic converter 208 may for example be an underbody catalystarranged to be a specified distance downstream of an exhaust port (notshown) of the engine.

The engine operation includes two major modes, and two transitionalmodes (although the engine need not necessarily operate under thesemodes at all times and other modes of operation are possible). Preferredmodal a operation of the engine is best shown in FIG. 4, which shows aload speed curve for engine operation. Engine load is represented asBreak Mean Effective Pressure (BMEP).

In lean operation mode (indicated by reference numeral A), the engine Iscalibrated to operate in lean burn mode, with a stoichiometriccoefficient of preferably greater than 1.3. (ie: The stoichiometriccoefficient is 1 for a stoichiometric air-fuel ratio, greater than 1 fora lean air-fuel ratio, and less than 1 for a rich air-fuel ratio.) Inthe stoichiometric ratio mode (indicated by reference numeral C), theair-fuel ratio is maintained at a substantially stoichiometric levelwith a stoichiometric coefficient of substantially 1.0. Preferablyexhaust gas is re-circulated to the combustion chambers to comprisegreater than 25% by mass of the gas in the chamber under lean modes ofoperation and preferably no greater than 40%. The amount of exhaust gasincreasing as the air fuel ratio gets leaner. Exhaust gas may also bere-circulated to the combustion chambers in stoichiometric modes ofoperation, however dual injection of fuel, as detailed further herein,is preferably employ

Engine operation is preferred in either one of these major modes ofoperation, however, a first transition mode (indicated by referencenumeral B) may be required when transferring between stoichiometric modeC and lean mode A. A transitional peak mode (indicated by referencenumeral D) may also be provided, and is used for specific high loadoperation for generally temporary operation using a fuel rich air-fuelratio (stoichiometric coefficient less than 1).

During the lean mode operation A, the temperature of the exhaust gas atthe entry to the main catalyst 208 is preferably in the range of 200 to400 degrees Celsius. In stoichiometric operation C, the temperature ofthe exhaust gas at the entry to the main catalyst 208 is typically above400 degrees Celsius. Conveniently, in this latter mode of operation, theengine can be controlled by way of a dual injection strategy such asthat disclosed in the Applicants' International Patent Application No.PCT/AU98/01004, the contents of which are included herein by reference.

Control of the system can be performed in two different ways. Firstly,the mode of the engine can be controlled on the basis of the known orestimated temperature of the exhaust gas. In this case, a sensor 214 canprovide information to the engine management system for the purposes ofcontrolling the engine operation appropriately. Secondly, thetemperature of the exhaust gas can be controlled to fit the mode ofoperation under which the engine is currently operating or is desired tooperate. Exhaust gas temperature may be controlled, for example, byvarying ignition timings from cycle to cycle (corresponding variationsof fuelling level may also be required). Of course, a combination ofthese two methods of control can also be used.

The main catalytic converter 208 is a three way converter whichcatalytically treats hydrocarbons, carbon monoxide gases and nitrousoxides. The Applicant has found that a Pt—Rh—Ba catalytic converter isparticularly useful, and specifically has found that the characteristicsof a Johnson-Matthey development version D2681/JM370 provides especiallygood results. This catalytic converter has a ratio of Pt:Rh of 10:1 inthe catalytically active part of the converter. The catalytic converteralso has a small proportion of Ba therein.

It is believed that the operation of the engine 200 in mode A so as topromote exhaust gases with a lean air fuel ratio and relatively lowergas temperatures supports a selective NOx reduction process that is nottypically supported by a normal 3 way catalyst. It is further believedthat this selective NOx reduction process is further supported by thepresence of a Pt rich catalytic converter, and perhaps still further bythe presence of some Ba on the converter. This selective NOx reductionprocess promotes the reduction of NOx emissions down to the lessharmiful components such as N₂O, N₂ and O₂. Alternate theory suggeststhat the Ba may, at least in part, provide NOx adsorption capabilities,and may even act as a catalyst commonly referred to as a Lean NOx Trap(LNT) or Lean NOx Catalyst (LNC). This allows some of the NOx to bestored for conversion into less harmful emissions when the engineoperates in mode C as described in greater detail herein.

In mode C, the engine 200 is controlled in such a way to take advantageof the high conversion efficiencies that the catalyst converter 208 canprovide under stoichiometric operating conditions, these conditionsbeing synonymous with higher exhaust gas temperatures and higher loadoperating by points.

The use of the close coupled catalytic converter 206 as illustrated inFIG. 3 can increase the effectiveness of the overall emission reductionprocess by oxidising hydrocarbon and carbon monoxide emissions underconditions which produce lower temperature exhaust gases (for example,the lean mode operation) as the temperature of the exhaust gasesimmediately adjacent the engine are significantly greater thandownstream at the main catalytic converter 208. The reason this isbeneficial is that these emissions (hydrocarbons and carbon monoxide)are more efficiently catalysed at higher temperatures. The combined leanstratified and stoichiometric NOx treatment according to the presentembodiment enables some of the potential problems of prior art systemsand in particular NOx storage type methods to be avoided as the catalystmay be purged of NOx by operating the engine under stoichiometricconditions.

In an alternate embodiment, a three way catalyst may be re-located froma close coupled position to an underbody position. An underbody positionis a position remote from the engine bay and associated fire wall, andis typically between the ground and the underside of the floor of thevehicle. In this instance the three way catalyst is preferable locatedin a position adjacent a catalyst having NOx adsorbent properties, suchas a catalyst having Ba as a constituent. Preferably, the catalysthaving NOx adsorbent properties operates additionally as a three waycatalyst. The three way catalyst that has been re-located to anunderbody position is preferably located in a single canister togetherwith the catalyst having NOx adsorbent properties. Preferably the threeway catalyst is located at the inlet of the canister and the catalystwith NOx adsorbent properties is located at the outlet of the canister.Locating the three way catalyst adjacent the inlet of the canisterallows the three way catalyst to be heated by the exhaust gasses emittedfrom the engine. This transfer of heat to the three way catalyst alsoserves to cool the exhaust before it flows through the catalyst with NOxadsorbent properties. In this way both the three way catalyst and thecatalyst with NOx adsorbent properties are generally maintained withintheir respective windows of operational temperatures. Some control ofthe engine may be required to achieve this. Specifically control ofvariables such as fuel per cycle and ignition timing may also beimplemented to maintain exhaust gas temperatures in a range sufficientto keep the catalysts in their operational temperature windows. As thethree way catalyst is now located in an underbody position it ispreferable that it is rapidly heated at starting of the engine. Suchheating being commonly referred to as a light off strategy and may beachieved through use of a heating element such as a resistive heatingelement or by use of exhaust gases as detailed in the Applicants U.S.Pat. No. 5,655,365 or any other suitable means. It has been found thatoptimum results may be achieved by location of the underbody catalyst adistance of between 1.0 m and 1.5 m along the exhaust system from theengine.

In a further embodiment, the three way catalyst and catalyst with NOxadsorbent properties form separate parts of the same three way catalystbrick. The catalyst with NOx adsorbent properties forming that part ofthe brick to which Ba is added.

With these arrangements, the catalyst with the NOx adsorbent propertiesmay be regenerated by operating the engine with a stoichiometric airfuel ratio (note: regeneration of a NOx adsorbent catalyst is oftenreferred to as “purging” the catalyst).

It is preferable that when operating the engine in mode A, ie lean mode,that the combustion chamber gas comprise 25% or more EGR by mass. EGRbeing an acronym for Exhaust Gas Re-circulation. EGR meansre-circulation of some of the exhaust gasses into the inlet manifold ofthe engine and hence into the combustion chambers of the engine.

Preferably the combustion chamber gases comprise between 25% and 40% EGRby mass with the percentage of EGR increasing as the air fuel ratioincreases (ie as the air fuel ratio gets more lean).

By maintaining the engine out NOx to a level of approximately twice theEuro III tail pipe emissions, the applicant has found the abovereferenced three way catalyst with NOx adsorbent properties to beparticularly effective. It is believed that with PGM (precious groupmetals—ie Pt, Pd, Rh etc) loadings that are relatively standard formanifold port injected vehicles, engine out NOx emissions of betweenthree and four times Euro III may be emitted whilst the catalyst willstill be effective for meeting Euro III requirements. Such a catalysthaving an engine swept volume (ESV) of less than 150% and preferablyless than 110%. It is believed that engine out CO emissions should atthe same time be in the order of three times or less Euro III emissionsin order to meet Euro III emissions requirements. Further it is believedthat the engine out HC emissions should be in the order of ten times orless Euro III emissions in order to meet Euro III emissions. Preferablythe engine is calibrated across its speed load range so that itsemissions do not to exceed these limits over a particular drive cycle.This may require that when the engine is operated in a lean mode thatthe air fuel ratio correspond with a lambda value no less than 1.3. Moreover as the lambda value increases, the ECR percentage should alsogenerally increase to a limit of approximately 40%. In somecircumstances, an air fuel ratio corresponding to a lambda of between1.0 and 1.3 may be selected when transitioning between a lean air fuelratio operating point and a stoichiometric air fuel ratio operatingpoint.

Selection of whether a load point should be lean or stoichiometric, andif lean, the limit to which it can be lean is generally determined foran engine during calibration. A trade off between lean operation, powerrequirements, NOx levels and levels of other emissions will be required.However, to meet Euro III and Euro IV requirements at least, it isbelieved that lean operating points should be calibrated to have ISNOx(Indicated Specific NOx) emissions levels in the range between 0.7 and2.0 grams per kilowatt hour in order for a three way catalyst with someNOx adsorbent properties to be utilised. It is believed that bycalibrating the engine so that the emissions are maintained in the abovebounds that PGM loadings similar to current MPI vehicles may beutilised. Optimally, the catalyst may have a size of less than 150% ESV(engine swept volume) and preferably less than 110% ESV. This range ofcalibration points is believed to provide optimum operation of an enginecapable of generating engine out NOx of approximately one and one halftimes Euro III levels, three times Euro III CO levels and ten times EuroIII HC levels. Calibration with lower NOx levels may be possible,however a larger three way catalyst may be required and fuel consumptionmay also deteriorate. Hence it is believed that the above range of ISNOxin combination with an exhaust treatment system having a three waycatalyst and a catalyst having some NOx adsorbent properties provides anoptimum configuration for meeting Euro III and/or Euro IV emissionstargets.

Selection between air fuel ratio and modes A, B, C and D is demonstratedwith reference to the dual mode strategy detailed in FIG. 5 which may beexecuted by an electronic control unit (ECU) of the engine. The dualmode strategy commences at step 500 whereupon it proceeds to step 505where the current gear of the vehicle is identified, typically, firstsecond, third, fourth or fifth for a manual passenger vehicle. Havingdetermined the current gear, the process proceeds to step 510 whichdecides to branch to step 515 if the gear identified is a low gear,typically first and second, and to branch to step 535 if the gear is ahigh gear, typically third gear or higher. At step 515 a variable E1,which is an engine load threshold value is set to a predetermined levelcorresponding to F_Low. This value indicates the boundary between modesB and C in FIG. 4. The process then proceeds to step 520 where itdetermines whether or not the engine is currently operating in an airled mode (typically stoichiometric or rich air fuel ratio andcorresponding to high load demand) or a fuel led mode (typically leanair fuel ratio corresponding to low load demand). If the engine isoperating in an air led mode then the process moves to step 530,otherwise it moves to step 525 and the value of E1 is reduced by anamount L1, which is a low gear hysteresis number which defines ahysteresis band for transitioning from an air led mode to a fuel ledmode (ie, a hysteresis for engine loads when moving from Mode C to ModeA) under low gear operating conditions, after which the process moves tostep 530.

Returning to step 510, if the vehicle is in a high gear then the processmoves to step 535 and the engine load threshold variable “E1” is set toF_High, being a high load value. The process then moves to step 540where it is determined whether or not the engine Is currently operatingwith an air led mode or a fuel led mode. If it is operating with an airled mode then the process moves to step 530, otherwise the process movesto step 550 where the engine load threshold value is reduced by the highgear hysteresis number which defines a hysteresis band for transitioningfrom an air led mode to a fuel led mode (ie, a hysteresis for engineloads when moving from Mode C to Mode A) under high gear operatingconditions, after which the process moves to step 530.

At step 530 the process determines whether or not the current engineload is greater than the current engine load threshold E1. If it is not,then the process moves to step 555 and a fuel led (or lean air fuelratio) is identified and the engine operates in mode A.

If at step 530 the current engine load is greater than the currentengine threshold value E1 then the process moves to step 565 andoperation an air led mode is identified. The process then moves to step570 where if the engine load is greater than engine threshold value E2then the engine operates in Mode D, which is a mode with rich air fuelratios. If however at step 570 the current engine load is identified asbeing less than E2 ten the process moves to step 580 which correspondswith Mode C, ie a stoichiometric air fuel ratio.

In preferred embodiments, an additional step 585 may be introducedintermediate step 570 and step 580. This step may determine whether ornot the exhaust gas is within a predetermined range, such as rangesuitable for efficient operation of a catalyst with NOx adsorbentcharacteristics. If it is within this range, then the process may thenoperate at additional step 590 in Mode B.

In a further embodiment, the catalyst with NOx adsorbent properties maybe regenerated at a sufficient rate when operating the engine with astoichiometric air fuel ratio (ie lambda=1.0) that saturation of thecatalyst can be avoided. This allows the engine to operate under typicaldriving conditions such that a NOx sensor may not be required. As suchthe air fuel ratio for engine load conditions may be selectedindependently of NOx stored on the catalyst or calculated as stored onthe catalyst. This is because the engine load will typically dictatestoichiometric or rich operating conditions from time to time. As such,this intermittent operation at these lower air fuel ratios, as occursunder typical vehicle operating conditions, will often be sufficient tomaintain the catalyst in a non-saturated state.

Alternately, the catalyst may be monitored, either directly by a NOxsensor or indirectly by some other means, such as an exhaust gastemperature sensor. Where it is monitored directly, the engine can beoperated by selecting a stoichiometric air fuel ratio from time to timeso as to ensure that the catalyst does not saturate. Such an arrangementhaving an advantage that the fuel economy is not greatly penalised asmay be the case where the engine is operate with a rich air fuel ratio.

Indirect monitoring of the NOx stored on the catalyst may be achieved bya cumulative measure of NOx emitted from the engine. This may beachieved by monitoring the engine operating conditions over a period oftime. For example the period of time that the engine has spent atvarious operating points. If it is known the amount of NOx that islikely to be emitted at these operating points then the amount of NOxcan be estimated. These operating points may be identified as either oneof engine speed or engine load or both. In these circumstances, theengine may be deliberately operated with a stoichiometric air fuelratio, even though a lean air fuel ratio may be sufficient for currentengine operating conditions, so as to regenerate the NOx adsorbentcatalyst.

Alternate methods of estimating when to have stoichiometric excursionfrom a lean mode of operation so as to regenerate the catalyst may beemployed. For example, the amount of time since a stoichiometricexcusion last occurred or the amount of time since the engine lastoperated with a stoichiometric operating condition for a period of timeto purge the catalyst of a significant proportion of the NOx adsorbedthereto.

The method according to the present invention is applicable to both twostroke and four stroke engines incorporating direct injection systemsand particularly those operation with a dual fluid fuel injectionsystem. Modifications and variations as would be deemed obvious to theperson skilled in the art are included within the ambit of the presentinvention.

1. A method of treating NOx emissions in the exhaust gas of an internalcombustion engine having catalyst means including at least a firstcatalyst converter capable of treating NOx, said converter including acombination of Pt, Rh and Ba elements wherein the ratio of Pt to Rh isabout 10:1 and the proportion of Ba is relatively low compared to theproportions of Pt and Rh, the method including operating the engine in afirst mode to promote a first set of conditions and in a second mode topromote a second set of conditions, wherein the first set of conditionsinclude exhaust gases at an exhaust gas temperature in the range of 200to 400 degrees Celsius, and wherein the first mode of operation includesoperating the engine with a lean air-fuel ratio, and the second mode ofoperation includes operating the engine with a stoichiometric air-fuelratio, the method further including controlling the operation of theengine during the first mode so as to promote a selective catalyst NOxreduction process at the first catalytic converter, and controlling thetemperature of the exhaust gas to be in the range of 200-400 degreesCelsius by operation of the engine to ensure effective operation of thefirst catalyst converter under the first mode of operation.
 2. A methodaccording to claim 1, wherein the second set of conditions includeexhaust gases at a temperature greater than 200 degrees Celsius.
 3. Amethod according to claim 2, wherein the exhaust gas temperature isgreater than 400 degrees Celsius.
 4. A method according to claim 1,including measuring the exhaust gas temperature at the first catalystconverter.
 5. A method according to claim 1, including controlling thetemperature of the exhaust gas temperature of the engine by appropriateoperation of the engine to ensure effective operation of the firstcatalyst converter under the second mode of operation.
 6. A methodaccording to claim 5, including controlling the exhaust gas temperatureto be greater than approximately 400 degrees Celsius.
 7. A methodaccording to claim 1, wherein the operation of the engine is controlledduring the first mode so as to generate the exhaust gas emissions havingcharacteristics that can support acceptable levels of NOx conversionwithin the first catalyst converter.
 8. A method according to claim 1,wherein the first catalyst converter includes a combination of Pd, Rhand Ba elements.
 9. A method according to claim 1, wherein theproportion of Pt is greater than for a typical three way catalyst.
 10. Amethod according to claim 1, including controlling the operation of theengine during the second mode so as to promote high NOx conversionefficiency levels within the first catalytic converter.
 11. A methodaccording to claim 10, including operating the engine in the first modewhen the sensed temperature is between 200 to 400 degrees Celsius, andoperating the engine in the second mode when the sensed temperature isgreater than 400 degrees Celsius.
 12. A method according to claim 1,wherein the catalyst means includes a second catalyst converter providedin a close coupled configuration with the engine for the purpose ofoxidizing hydrocarbon and carbon monoxide emissions in the exhaust gas.13. A method according to claim 1, wherein the first catalyst converteris a three way catalyst.
 14. A method according to claim 1, wherein theengine is directed injected.
 15. A method according to claim 14, whereinthe engine has a two fluid fuel injection system.
 16. An engine exhaustsystem for treating NOx emissions in the exhaust gas of an internalcombustion engine, including catalyst means having at least a firstcatalyst converter capable of treating NOx, said converter including acombination of Pt, Rh and Ba elements wherein the ratio of Pt to Rh isabout 10:1 and the proportion of Ba is less than the proportions of Ptand Rh, wherein the engine exhaust system is adapted to at leastselectively reduce a portion of the NOx emissions when the engine isoperated in a first mode and a first set of conditions are promoted,wherein the first set of conditions include exhaust gases at an exhaustgas temperature in the range of 200 to 400 degrees Celsius, and thefirst mode of operation includes operating the engine with a leanair-fuel ratio, and controlling the temperature of the exhaust gas to bein the range of 200-400 degrees Celsius by operation of the engine toensure effective operation of the first catalyst converter under thefirst mode of operation.
 17. An engine exhaust system as claimed inclaim 16 for use with direct injection engine whereby said first mode ofoperation is promoted.
 18. An engine as claimed in claim 17 wherein saiddirect injection engine utilizes an air assisted direct injection fuelsystem.
 19. An engine operating system according to claim 16, whereinthe first catalyst converter includes a combination of Pd, Rh and Baelements.
 20. An engine exhaust system according to claim 16, whereinthe proportion of Pt is greater than for a typical three way catalyst.21. An engine exhaust system according to claim 16, including atemperature sensing device provided in the exhaust system of the enginefor measuring the exhaust gas temperature.
 22. An engine exhaust systemaccording to claim 21, wherein the temperature sensing device is locatedat the first catalyst converter.
 23. An engine exhaust system accordingto claim 21, wherein the engine is operated in the first mode when thesensed temperature is between 200 to 400 degrees Celsius, and the engineis operated in the second mode when the sensed temperature is greaterthan 400 degrees Celsius.
 24. A method according to claim 16, whereinthe catalyst means includes a second catalyst converter provided in aclose coupled configuration with the engine for the purpose of oxidizinghydrocarbon and carbon monoxide emissions in the exhaust gas.
 25. Amethod according to claim 16, wherein the first catalyst converter is athree way catalyst.
 26. An internal combustion engine in combinationwith the exhaust system according to claim 16, wherein said engine has afuel injection system which facilitates operation of said engine with aplurality of air fuel ratios in a range between lean and substantiallystoichiometric and said engine having an electronic controller forcontrolling operation of said engine and for selecting saidsubstantially stoichiometric air fuel ratio to purge Nox stored in saidexhaust treatment system, wherein said electronic controller selects andstoichiometric air fuel ratio at least as a cumulative measure ofemissions transmitted to the exhaust treatment system.
 27. An internalcombustion engine as claimed in claim 26 wherein said cumulative measureis determined from engine operating conditions over a predeterminedperiod of time.
 28. An internal combustion engine as claimed in claim 27wherein said operating conditions is at least one of engine speed and/orengine load.
 29. An internal combustion engine as claimed in claim 27where said predetermined period of time is elapsed time since saidengine operated with a stoichiometric air fuel ratio.
 30. An internalcombustion engine as claimed in claim 29 wherein said predeterminedperiod of time is elapsed time since said engine operate with astoichiometric air fuel ratio for a period sufficient to substantiallypurge said catalyst of stored NOx.
 31. An internal combustion engine asclaimed in claim 26 wherein said cumulative measure is an estimate basedon emission levels emitted at each selected air fuel ratio.
 32. Aninternal combustion engine as claimed in claim 26 wherein saidcumulative measure is based on the amount of time said engine wasoperated at each selected air fuel ratio.
 33. An internal combustionengine as claimed in claim 26 wherein said stoichiometric air fuel ratiois selected for a period sufficient to regenerate said exhaust treatmentsystem from stored NO, and wherein subsequent to said period sufficientto regenerate said exhaust treatment system said electronic controllerselects an air fuel ratio dependent on prevailing engine conditions. 34.An internal combustion engine as claimed in claim 26 wherein saidelectronic controller select said stoichiometric air fuel ratio inresponse to a sensing means operatively arranged with respect to theexhaust treatment system which is able to provide an indication on theamount of NOX stored therein.
 35. An internal combustion engine asclaimed in claim 34 wherein said electronic controller only selects saidstoichiometric air fuel ratio in response to a signal from said sensingmeans that purging of NOx from the exhaust treatment system is required.36. An internal combustion engine as claimed in claim 34 wherein saidselection of said stoichiometric air fuel ratio by the electroniccontroller to effect purging of NOX from the exhaust treatment system isalso dependent on the volume of a catalyst in the exhaust treatmentsystem.
 37. An internal combustion engine as claimed in claim 26 whereinsaid engine is a direct injection engine.
 38. An internal combustionengine as claimed in claim 26 wherein said engine is a dual fluid directinjection engine.
 39. An internal combustion engine and exhausttreatment system as claimed in claim 26 said exhaust treatment systemcomprising at least one catalyst having three way conversion capabilityand NOx storage capability, wherein the amount of NOx emitted by saidengine to said exhaust treatment system over a Euro III drive cycle isno more than four times the Euro III requirement whereby said exhausttreatment system has emissions of NOx, carbon monoxide and hydrocarbonsless than said Euro III requirement over said Euro III drive cycle, andthe volume of the catalyst is less than 150% of the swept volume of saidengine.
 40. An internal combustion engine and exhaust treatment systemas claimed in claim 39 wherein said catalyst has substantially twozones, a first of which has said three way conversion capability and asecond of which has at least said NOx storage capability.
 41. Aninternal combustion engine and exhaust treatment system as claimed inclaim 40 wherein said second zone of said catalyst has three wayconversion capability in addition to said NOx storage capability.
 42. Aninternal combustion engine and exhaust treatment system as claimed inclaim 40 wherein said first zone is located so as to received exhaustemissions from said engine before said second zone.
 43. An internalcombustion engine and exhaust treatment system as claimed in claim 39wherein said exhaust treatment system has a single canister for locatingsaid at least one catalyst, said canister located remotely from anexhaust port of said engine and not within an engine compartment inwhich the engine is installed.
 44. An internal combustion engine andexhaust treatment system as claimed in claim 43 wherein single canisteris located in an underbody location and has dimensions of less than 150%of the swept volume of the engine.
 45. An internal combustion engine andexhaust treatment system as claimed in claim 39 wherein exhaustemissions generated by said engine when operated with a substantiallystoichiometric air fuel ratio operate to purge NOx stored in saidexhaust treatment system during said Euro III drive cycle.
 46. Aninternal combustion engine and exhaust treatment system for a vehicle asclaimed in claim 39 wherein the amount of carbon monoxide emitted bysaid engine to said exhaust treatment system over said Euro III drivecycle is no more than three times the Euro III requirement.
 47. Aninternal combustion engine and exhaust treatment system as claimed inclaim 39 wherein the amount of hydrocarbons emitted by said engine tosaid exhaust treatment system over said Euro III drive cycle is no morethan ten times the Euro III requirement.
 48. An internal combustionengine and exhaust treatment system as claimed in claim 39 wherein theamount of NOX emitted by said engine to said exhaust treatment systemover said Euro III drive cycle is no more three times the Euro IIIrequirement.
 49. An internal combustion engine and exhaust treatmentsystem as claimed in claim 39 wherein for substantially all of the leanair fuel ratios, said engine operates with EGR levels of 25% by mass orgreater.
 50. An internal combustion engine and exhaust treatment systemas claimed in claim 39 wherein in operation said catalyst is heated byalight off strategy.
 51. An internal combustion engine and exhausttreatment system as claimed in claim 50 wherein said light off strategycomprises late combustion of fuel whilst an exhaust port of said engineis open whereby said catalyst receives exhaust emissions of an elevatedtemperature.
 52. An internal combustion engine and exhaust treatmentsystem as claimed in claim 51 wherein late combustion of fuel comprisesa quantity of fuel in addition to a quantity required for operation ofsaid engine independent of said light off strategy.
 53. An internalcombustion engine and exhaust treatment system as claimed in claim 39wherein said engine is a direct injection engine.
 54. An internalcombustion engine and exhaust treatment system as claimed in claim 39wherein said engine is a dual fluid direct injection engine.
 55. Amethod according to claim 1, wherein the engine operates in said firstmode with EGR levels of 25% by mass or greater.
 56. A method accordingto claim 1, wherein the catalyst is heated by a light off stategy.
 57. Amethod according to claim 56, wherein said light off strategy compriseslate combustion of fuel whilst an exhaust port of said engine is openwhereby said catalyst receives exhaust emissions of an elevatedtemperature.